Metal belts for use in continuously variable transmissions comprise a plurality of metal rings laminated together which have slightly different circumferential lengths. Heretofore, such a metal belt is manufactured as follows:
First, a thin sheet of ultrahigh strength steel such as maraging steel is bent into a loop, and has its opposite ends welded to each other, producing a ring-shaped drum. The drum is then sliced into metal rings of certain width. The metal rings are rolled into metal rings having standard circumferential lengths suitable for the respective layers of the metal belt. In this manner, as many kinds of metal rings as the number of the layers of the metal belt are obtained.
When the metal rings in those kinds are then subjected to a solution treatment, the circumferential lengths of the metal rings which have been rolled to the standard circumferential lengths suitable for the respective layers of the metal belt are varied due to different treating conditions. Therefore, the metal rings subjected to the solution treatment are treated in a circumferential length correcting process to correct their circumferential lengths into the standard circumferential lengths suitable for the respective layers of the metal belt.
The metal rings which have been treated in the circumferential length correcting process are then aged and nitrided to increase their hardness. The metal rings of the kinds having their circumferential lengths, which are slightly different from each other, corrected into the standard circumferential lengths suitable for the respective layers of the metal belt according to the circumferential length correcting process are laminated together, thus producing a metal belt for a continuously variable transmission.
When the metal belt is thus manufactured, if the variations of the circumferential lengths which have been caused by the solution treatment are left uncorrected, then many metal rings remain uncombinable with other metal rings at the time metal rings of different kinds are laminated together. Since the ultrahigh strength steel such as maraging steel is expensive and hence metal rings need to be manufactured with as high a yield as possible, the circumferential length correcting process is highly important for producing metal belts for continuously variable transmissions.
The applicant has already proposed an apparatus for performing the above circumferential length correcting process as disclosed in Japanese laid-open patent publication No. 11-290971. As shown in FIG. 15 of the accompanying drawings, the disclosed circumferential length correcting apparatus 101 has a drive roller 102 and a driven roller 103 for training a metal ring W therearound, and a correcting roller 104 disposed in a position intermediate between the drive roller 102 and the driven roller 103.
The drive roller 102 is rotatably supported by a drive roller support member 106 fixedly mounted on a base 105 of the circumferential length correcting apparatus 101. The drive roller 102 is coupled by a coupling mechanism, not shown, to a drive motor 107 which is a rotational drive source disposed behind the base 105. A guide block 109 is horizontally slidably mounted in engagement with a guide rail 108 fixedly mounted on the base 105. The guide block 109 supports thereon a driven roller support member 110 on which the driven roller 103 is rotatably supported.
The drive roller support member 106 and the driven roller support member 110 have a pair of spacers 111, 112 disposed respectively thereon and held in abutment against each other for spacing the drive roller 102 and the driven roller 103 from each other by a given interaxial distance. To the driven roller support member 110, there is connected one end of a wire 113 extending horizontally, whose other end is connected to counterweights 114a, 114b. The counterweights 114a, 114b are suspended vertically by rolls engaged by the wire 113. After the metal ring W is trained around the drive roller 102 and the driven roller 103, the driven roller 103 is displaced in a direction away from the drive roller 102 under the load of the counterweights 114a, 114b. 
A frame 115 is vertically mounted on an end of the base 105 near the drive roller 102, and supports a hydraulic cylinder 116 on its upper portion. The correcting roller 104 is rotatably supported by a correcting roller support member 118 which is mounted on the end of a piston rod 117 of the hydraulic cylinder 116. The correcting roller 104 is displaceable by the hydraulic cylinder 116 in directions (vertical directions) perpendicular to the directions in which the driven roller 103 is displaceable.
A support column 119 is vertically mounted on an opposite end of the base 105 near the guide rail 108, and supports a first displacement sensor 120 for detecting displacements of the driven roller 103. A second displacement sensor 121 for detecting displacements of the correcting roller 4 is mounted on the frame 115 which supports the hydraulic cylinder 116.
The conventional circumferential length correcting apparatus 101 shown in FIG. 15 operates as follows: With the driven roller 103 brought closely to the drive roller 102, the metal ring W is trained around the drive roller 102, the driven roller 103, and the correcting roller 104. While the drive roller 102 is being rotated by the drive motor 107, the driven roller 103 is displaced in the direction away from the drive roller 102 under the load of the counterweights 114a, 114b, thus tensioning the metal ring W. The displacement sensor 120 now detects a displacement of the driven roller 103. The interaxial distance between the drive roller 102 and the driven roller 103 is determined from the detected displacement, and the actual circumferential length of the metal ring W is calculated as a function of the interaxial distance. The circumferential length correcting apparatus 101 calculates a displacement of the correcting roller 104 which is required to correct the circumferential length of the metal ring W into a desired circumferential length, from the difference between the determined actual circumferential length of the metal ring W and the desired circumferential length.
Then, while the drive roller 102 is being rotated, the hydraulic cylinder 116 urges and displaces the correcting roller 104 upwardly, plastically deforming the metal ring W.
At this time, the correcting ring 104 urges the correcting roller 104 upwardly until the displacement of the correcting roller 104 which is detected by the displacement sensor 121 reaches the calculated displacement of the correcting roller 104, after which the correcting ring 104 releases the correcting roller 104.
Then, the circumferential length correcting apparatus 101 displaces the driven roller 103 again in the direction away from the drive roller 102, and calculates the actual circumferential length of the metal ring W in the same manner as described above. The circumferential length correcting apparatus 101 determines the difference between the actual circumferential length after it has been corrected and the desired circumferential length. If the actual circumferential length as corrected is in conformity with the desired circumferential length, then the above operation is put to an end.
If the actual circumferential length as corrected is not in conformity with the desired circumferential length, then the above operation is repeated based on the actual circumferential length and the desired circumferential length. The circumferential length of the metal ring W can thus be corrected into the desired circumferential length.
In the circumferential length correcting apparatus 101, the correcting roller 104 may be displaced a certain distance with respect to the circumferential length, which is assumed to be substantially constant, of the metal ring W after it has been rolled and subjected to the solution treatment. In this case, the circumferential length of the metal ring W which is measured after the metal ring W has been trained around the drive roller 102, the driven roller 103, and the correcting roller 104 is compared with a standard circumferential length that is defined for the purpose of design or process management as the circumferential length of the metal ring W after it has been rolled and subjected to the solution treatment. The displacement of the correcting roller 104 is corrected based on the difference between the compared circumferential lengths.
By thus correcting the displacement of the correcting roller 104, it is possible to correct the actual circumferential length of the metal ring W easily and reliably into a desired circumferential length with the correcting roller 104 being displaced only once, and the yield of corrected metal rings can be increased.
With the above circumferential length correcting apparatus 101, however, for training the metal ring W around the drive roller 102, the driven roller 103, and the correcting roller 104, the driven roller 103 must be manually brought toward the drive roller 102. The driven roller 103 must also be manually brought toward the drive roller 102 for removing the metal ring W from the drive roller 102, the driven roller 103, and the correcting roller 104. Consequently, it has been desirous to fully automatize all the steps of operation of the circumferential length correcting apparatus 101.